1. Field of the Invention
The present invention relates to thin-film coatings of flat substrates such as semiconductor wafers and flat panel displays, and in particular to application of photoresist and similar high-viscosity chemicals with minimized chemical waste and improved coating uniformity.
2. Description of the Related Art
In the fabrication of semiconductors, a wafer is coated with photoresist or a similar high-viscosity liquid chemical by any of a number of methods. (As used herein, the term "liquid" is not limited to fluids of low-viscosity and, in fact, can refer to highly viscous chemical substances.) These methods include dry film thermal application, spin coating, roller coating, meniscus coating, slot coating, spray coating, and silk screening. The most successful and popular of these methods is spin coating, in which photoresist is applied to the wafer and the wafer is spun to completely coat the wafer. The rate of rotation of the wafer is then increased, or "ramped up", to spin off excess photoresist to provide a coating of a desired thickness.
Until recently, spin coating has been mostly confined to coating circular surfaces or small square mask plates. However, spin coating is now used to apply chemicals to thin film heads, multi-chip modules, and flat panel displays which are square or rectangular and are often very large. Spin coating such surfaces presents primarily two problems.
First, the corners of a square or rectangular surface are poorly covered. The chemical being applied to the surface is spun to the edge where excess is thrown from the surface by centrifugal force. Chemical leaving the center of an edge is struck by the corner of the spinning surface as the corner is further from the center of the surface than is the center of an edge. Such a collision disrupts the even flow of chemical from the center of the surface to the corners of the surface.
Secondly, spin coating larger surfaces results in striations in the chemical coating toward the edges and corners of the surface as a result of uneven drying of the chemical. To cover a large surface before the chemical dries, the chemical is applied to a surface spinning at a normal speed and the speed of the spinning surface is then ramped up, i.e., gradually increased, as the chemical spreads. Increasing the spin rate spreads the chemical more quickly and potentially before the chemical thickens substantially.
The surface tension of the chemical on the spinning surface is what causes the chemical to spread evenly during spinning. However, as the spinning rate is increased, the centrifugal force overcomes the surface tension of the chemical. This is especially true on larger surfaces such as large flat panel displays or when small volumes of chemical are used. When the surface tension is overcome, the smooth circular shape of the spreading chemical bursts like a bubble and the chemical then streams linearly toward the edges of the spinning surface in multiple radial paths. These multiple radial paths contribute to the formation of striations in the coating of the spinning surface.
The radial gaps in coating, i.e., the gaps between the radial streams of chemical, are filled in by applying an excess of the chemical to the center of the spinning surface. Excess chemical is applied until radial flow of the chemical from the center of the surface, as a result of the extreme centrifugal force, fills in the uncoated area. Thus, the surface is coated by saturating the surface with chemical, most of which, i.e., 95% or more, is discarded as waste. Furthermore, the uniformity of the coating can be poor as a result of uneven coating when the surface is initially coated and uneven drying of the chemical coating.
Excessive use of photoresist in particular is a significant problem in the art. Photoresist accounts for approximately 5% of the cost of materials for semiconductors and generally costs as much as $1,000 or more per gallon. Much of the photoresist used to coat a wafer is wasted. For example, a 1 micron thick coating of photoresist with a 20% solids content on a 200 mm silicon wafer should require 0.16 cc of photoresist. However, to spin coat a 200 mm wafer typically uses 4 cc to 5 cc of photoresist; 97% of the photoresist is wasted. Thus, excessive waste of photoresist significantly affects the cost of manufacturing semiconductor devices. Lesser volumes are typically not used because, with lesser volumes, the photoresist dries prematurely and the uniformity of the coating is adversely effected.
Additionally, disposal of photoresist waste presents a substantial environmental burden on communities in which semiconductor devices are manufactured and on surrounding ecological systems. The problem of excess photoresist is exacerbated when coating larger surfaces.
What is needed is a method and apparatus which efficiently and effectively coats a substantially flat surface with a chemical. A method and apparatus capable of efficiency and effectively coating large, rectangular, flat surfaces with chemical is also desirable.